Marigold

Marigolds, in the context of this article, are plants of the genus Tagetes. While a good choice for your first forray into seed collecting, many commercial suppliers create hybrid plants which do not produce viable seed. When purchasing for seed production, avoid any marigolds labeled as hybrids.

Pollination
Marigolds require cross-pollination, usually provided by insect visitors.

Seed Collection
    Wait for individual flower heads to become completely dry before collecting. Your plant will have many heads at different ages, check regularly and collect any heads that are brittle and snap off easily. Many of the seeds will fall right out, but you'll likely need to rub the head open to get them all.